Methods of producing preforms from powdered metal or ceramic by cold isostatic compression are known in the art.
In one method, the powder is introduced into an elastic mold (for example, of rubber) and is compressed at a hydrostatic pressure of 500 to 6000 bars. By the insertion of cores it is also possible to produce an inner contour for the molded part.
Complex outer contours, for instance, of turbine wheels, cannot be produced by the known methods. This is because the elastic mold springs back upon the removal of the pressure and still rests against the molded body. As a result, forces directed radially outwards and/or axially are transmitted to the preform and may lead to the destruction of delicate regions of the preform, such as thin-walled regions.
DE-OS No. 14 83 684 discloses a method of manufacturing molded parts, particularly parts of complex outer contours, by cold isostatic compression in which an impermeable thin-walled elastic envelope is applied against the inner wall surface of a divisible outer supporting mold part of porous material by subjecting the external supporting mold part to a vacuum, whereafter the elastic envelope is filled with pulverulent molding material and closed and then subjected to cold isostatic compression pressure.
The advantage of this method is that high precision of shape can be obtained without inordinately high expenses for the molding. The thin-walled elastic envelope filled with the pulverulent molding material, however, has no inherent stiffness of its own so that the supporting mold parts are required during the cold isostatic compression step to assure the desired outer contour of the molded part. However, this imposes considerable limitations as regards the outer contours which can be produced. Thus, for instance, molded parts which have greatly curved contours with undercuts cannot be produced by the aforementioned method since, upon the contraction which takes place during the course of the isostatic compression there would be contact with the outer supporting mold. However, even if contact with the outer supporting mold, as a result of the contraction due to molding was not a concern, there would nevertheless be the danger that, since the outer supporting mold does not simultaneously contract, forces leading to the formation of cracks are produced in the molded part to be compressed.